METAZOA ECHINODERMA. 185 



margin (No. 343, b), while the madreporic body occupies 

 the usual central position of the anal area as seen in the 

 regular sea urchins. No. 343, g, has been bleached by 

 nature and through the microscope the openings of the 

 madreporic body are clearly seen. 



As the sand-dollar grows older it becomes flattened 

 and more or less heart-shaped (No. 343, h 1). The 

 petals become distinct, the outer ends are open, and the 

 pores extend towards the ambitus, but do not reach the 

 edge. At this time the internal partitions radiate in five 

 pairs from the edge toward the center and there are few 

 pillars. 



The ambulacra and interambulacra are often distinctly 

 seen in these younger stages, as in No. 343, m and n, 

 where the limits of the individual plates can be easily 

 traced ; those on the lower surface are much more 

 irregular than those above. The ambulacral furrows are 

 simple but branch at their outer extremities. Sometimes, 

 as in No. 343, p. there is no indication whatever of these 

 furrows. 



The ocular openings at the ends of the ambulacra are 

 large, but the genital plates and pores cannot be detected. 



The adult (No. 343, r, s) is more flattened than any of 

 the Clypeastroids and is covered by tiny, dark colored 

 spines (No. 343, r). It is usually difficult to make out 

 the ambulacral and interambulacral plates, but if the 

 specimen is bleached by nature, or treated with acid as is 

 the case with No. 343, s, they come out more clearly. 

 They are also finely seen from the inside when the dorsal 

 side is removed. The ambulacral furrows are well 

 defined and branch a few times. The petals above are 

 large with furrows between the holes ; they open at their 

 outer ends in very flat sand-dollars while in more convex 

 specimens they nearly converge. A few pairs of pores 

 extend downward from the petals towards the edge 

 (No. 343, s). 



The anus has moved downward to the edge (No. 343, 



